Graphic Instruments

Author: E.E. Kimberly

Graphic instruments draw curves for a permanent record of the variation of current, voltage, power, power factor, etc. with time. For every electrical indicating instrument, there is a corresponding graphic or curve-drawing instrument. These instruments are highly useful in analyzing factory loads and loads of individual machines. A graphic voltmeter and ammeter may show that a motor overheats because the line voltage is allowed to become too low and the current therefore becomes too great.

A graphic wattmeter may show that the load on a motor reaches the full-load value of the motor rating for only 10 minutes in every hour and does not exceed half-load in the other 50 minutes. An induction motor so loaded runs at a lower power factor and efficiency, and should be replaced with one that is nearer the correct size and represents less invested capital.